(1) Field
The embodiment(s) discussed herein is directed to a MIMO communication system and a transmission station. The embodiments are preferable in the case in which, for example, a preceding multi-input multi-output (MIMO) system is applied to an uplink communication in a mobile wireless communication system.
(2) Description of Related Art
A preceding MIMO system has been examined in an long term evolution (LTE) specification examined in 3rd generation partnership project (3GPP) as a third generation mobile wireless communication system. Following Japanese Patent Application Laid-Open No. 2007-110664 described below also describes the preceding MIMO system.
In the precoding MIMO system, predetermined a plurality of sets of preceding matrix (PM) (hereinafter referred to as “code book”) are defined. A reception station selects the preceding matrix, by which the most excellent reception property is achieved, by using a known signal (reference signal), and informs this information (index) of the transmission station. The transmission station selects the preceding matrix based on the index informed from the reception station, multiplies the transmission signal by the preceding matrix (the element of the preceding matrix) (as a weighing factor), and transmits the resultant. Thus, the throughput of the wireless communication from the transmission station to the reception station can be enhanced.
In the preceding MIMO system, the optimum number of data pieces (number of streams) simultaneously transmitted from the transmission station to the reception station can be selected. The number of streams may sometimes be referred to as a rank, and the selection of the optimum rank may sometimes be referred to as a rank adaptation.
In the LTE specification (see the following 3GPP TS36.211 V8.1.0 (2007 Dec. 20)), for example, in case where the number of transmission antennas is four, four ranks of 1, 2, 3 and 4 are defined, wherein 16 types (64 types in total) of PM are defined for the corresponding ranks, in relation to the downlink (DL) communication in the direction from a base station to an user equipment (UE).
The UE selects the PM, by which it is estimated that the most excellent reception property is obtained, among 64 types of PMs, and reports this number (index) to the base station. The base station carries out a downlink transmission by using the PM having the reported number.
In case where the number of the transmission antennas is two, 9 types of PMs in total are defined in the LTE specification (see 3GPP TS36.211 V8.1.0 (2007 Dec. 20)). Specifically, the set of PM in the case of the rank 1 is expressed by the following equation (1), and the set of PM in the case of the rank 2 is expressed by the following equation (2), respectively. Notably, VL,M,N represents the Nth PM in the combination (code book) of the PM of L×M matrix, when the number of the transmission antennas is defined as L (L is an integer of 2 or more), and the number of transmission streams (ranks) is defined as M (M is an integer of 1 or more)
                                          V                          2              ,              1              ,              0                                =                      [                                                            1                                                                              0                                                      ]                          ,                              V                          2              ,              1              ,              1                                =                      [                                                            0                                                                              1                                                      ]                          ,                              V                          2              ,              1              ,              2                                =                                    1                              2                                      ⁡                          [                                                                    1                                                                                        1                                                              ]                                      ,                                  ⁢                              V                          2              ,              1              ,              3                                =                                    1                              2                                      ⁡                          [                                                                    1                                                                                                              -                      1                                                                                  ]                                      ,                              V                          2              ,              1              ,              4                                =                                    1                              2                                      ⁡                          [                                                                    1                                                                                        j                                                              ]                                      ,                              V                          2              ,              1              ,              5                                =                                    1                              2                                      ⁡                          [                                                                    1                                                                                                              -                      j                                                                                  ]                                                  •••                                                V                          2              ,              2              ,              0                                =                                    1                              2                                      ⁡                          [                                                                    1                                                        0                                                                                        0                                                        1                                                              ]                                      ,                              V                          2              ,              2              ,              1                                =                                    1              2                        ⁡                          [                                                                    1                                                        1                                                                                        1                                                                              -                      1                                                                                  ]                                      ,                              V                          2              ,              2              ,              2                                =                                    1              2                        ⁡                          [                                                                    1                                                        1                                                                                        j                                                                              -                      j                                                                                  ]                                                  •••      
The 2×2 matrixes in the equation (2) are unitary matrixes, and the 2×1 matrixes in the equation (1) correspond to the extracted column components in each unitary matrix. Since the value by which the total transmission power becomes 1 is multiplied as a normalized coefficient, the 2×1 matrixes are those of constant multiple of the unitary matrixes, to be correct.
Similarly, when the number of the transmission antennas is four, 4×4 matrixes (16 types) in the case of rank 4 are the unitary matrixes, and 4×3 matrixes in the case of rank 3, 4×2 matrixes in the case of rank 2, and 4×1 matrixes in the case of rank 1 respectively correspond to the extracted column components in each unitary matrix. Specifically, the matrixes of PM other than the rank 1 are mutually orthogonal.
In a cellular system, reducing power consumption of UE is important. The enhancement in the power efficiency of an amplifier for a transmission signal of the UE is effective for reducing the power consumption. When the power efficiency of the amplifier is considered from the viewpoint of the transmission signal, it is desirable that the peak to average power ratio (PAPR) of the transmission signal is reduced.
An orthogonal frequency division multiplexing (OFDM) system has been proposed as a wireless access system that is strong for a frequency selective fading in a multi-pass in a broadband radio communication. However, this system employs a multi-carrier transmission, so that the PAPR of the transmission signal tends to increase. From the viewpoint of the power efficiency of the UE, it is unsuitable for the transmission system of the uplink (wireless link from the UE to the base station) in the cellular system.
Therefore, as the transmission system of the uplink (UL) in the LTE specification, a system described below has been proposed. Specifically, in this system, a transmitter adds a cyclic prefix (CP) to an effective symbol in a time-domain to carry out a single carrier transmission, and a receiver performs a frequency equalization. Examples of this system include an single carrier frequency division multiple access (SC-FDMA) system.
In the single carrier transmission, transmission data signals or known signals (reference signal or pilot signal) between the transmission and reception are multiplexed in the time-domain, so that it can suppress the PAPR, compared to the OFDM in which the data signals or known signals are multiplexed in the frequency region.
Since a power added efficiency of an amplifier is enhanced as an output power thereof increases, it is desirable that an operating point is made close to the maximum value of the output power as much as possible. However, when the output power exceeds a fixed threshold value (saturation power), a non-linear distortion, which is non-tolerable as a transmission signal, might occur. Therefore, there is a trade-off relationship between the distortion and the power added efficiency.
As the PAPR of the transmission signal is small, the difference (back-off) between the operating point and the threshold value can be decreased (e.g., see FIG. 3). The PAPR is mostly used as an index of evaluating the back-off necessary for the design of the amplifier, but an evaluation index of raw Cubic Metric (raw CM) represented by the following equation (3) is also proposed in the following Motorola, “Comparison of PAR and Cubic Metric for Power De-rating” (R1-040642, TSG RAN WG1 #37), 2004. 5 described below. The relative back-off can be defined by the following equation (4) by using the raw CM.rawCM=20*log 10((v_norm3)rms)  □□□CM=[rawCM−20*log 10((v_norm_ref3)rms)]/M  □□□
The value obtained from the equation (4) is referred to as Cubic Metric (CM). This value is close to the actual value compared to the back-off calculated by using the PAPR. In the equations (3) and (4), v_norm represents an amplitude of the normalized input signal, while v_norm_ref represents an amplitude of a signal that becomes a reference. Further, ( )rms means that the root mean square is assumed, and M is a value determined by the property of the amplifier.
In the existing LTE specification (3GPPTS36.211V8.1.0 (2007 Dec. 20)), the application of the preceding MIMO system is only examined with respect to the downlink (DL) communication (multi-carrier transmission), in which the increase of the PAPR is not a problem, compared to the UE.